This invention relates to devices for static noise testing of jet aircraft engines. In particular, it relates to devices for attenuating turbulence in air entering the intake of a jet engine.
The advent of larger jet engines during a time of increasing concern over noise pollution has led to intensified searches for ways to reduce noise emission from jet aircraft engines. The design and development of quieter engines has necessitated extensive static noise testing. To be meaningful, static noise tests must be conducted under conditions which accurately reproduce engine noise characteristics as they exist during actual in-flight operation.
It has been found that a primary source of noise emission from turbofan jet engines is associated with the flow of air through the stationary and moving blades of the fan stage. In particular, it is found that the highest noise levels occur at a frequency known as the blade passing frequency, and at the higher harmonic orders thereof. The blade passing frequency may be defined as the arithmetic product obtained by multiplying the rotational speed of the engine, in revolutions per second, by the number of rotating fan blades. The blade passing frequency, as its name implies, is therefore the number of blades passing by any fixed point on the engine nacelle per second.
In practice, it is found that engine noise levels produced at the blade passing frequency and higher order harmonics thereof are ten to twenty decibels higher than noise levels at other frequencies. Thus, it is clear that overall noise reduction can best be achieved by reducing emission of noise at the blade passing frequency and its harmonics. Noise reduction research has, accordingly, focused on this problem.
It is also found, however, that the noise emitted from any particular engine differs according to whether the noise is measured during a static test or during in-flight operation. In fact, a difficulty encountered in attempting to make realistic static noise tests on the ground lies in the fact that such tests are characterized by higher absolute noise levels at the blade passing frequency than those attending in-flight engine operation. This disparity has been particularly acute with respect to high bypass ratio turbofan engines, such as those used on Boeing 747 airplanes. It is known that a significant part of this difference between static and in-flight noise emission is due to steady and unsteady distortions, or turbulence, in the intake air entering the turbofan inlet during static tests. Tests show that atmospheric turbulence is ingested by the turbofan and that the turbulence becomes increasingly anisotropic due to flow contraction as the air approaches the inlet face. In the vicinity of the fan the intensity of the turbulence velocity components transverse to the direction of airflow may be as high as five percent of the mean flow velocity. It is primarily the interaction of the fan rotors with this turbulence which in ground testing produces the abnormal non-representative noise levels at the blade passing frequency and its higher harmonics. Corresponding atmospheric turbulence effects during flight are negligible.
In order to begin to solve the general problem of reducing in-flight aircraft noise levels at the blade passing frequency, it has therefore been found necessary to create experimental conditions during static ground tests which more nearly reproduce the air intake conditions characteristic of in-flight engine operation. Several experimental engine inflow control devices have been constructed in the past with a view toward reducing intake air turbulence. These devices have proven less satisfactory than the present invention for several reasons. First, the devices were mounted on large external support structures that were located at least partially forward of the inlet highlight plane. Such structures distorted the sound emitted through the control surface by reflection and diffraction.
Secondly, the past devices generally were hemispherical in shape, the hemisphere generally capping the engine intake and being closed around the engine by a flat annular baffle extending in a plane normal to the engine axis between the outer surface of the engine intake and the aft edge of the hemisphere. It has been found that such a baffle has a significant effect on the sound pressure field radiating forwards from the engine intake.
Further, the hemispherical shape of the past devices was only crudely approximated by an assemblage of a relatively small number of large, flat flow panels, hence providing relatively sharp dihedral corners where the panels join. These corners create "dead spaces" which were found to distort sound incident upon them. Also, the hemispherical enclosure framework of the earlier models was constructed of rather large frame members which in themselves occupied and obstructed flow through a relatively large percentage of the hemispherical surface area and also adversely distorted the emitted engine sound.
Finally, the dimensions of the honeycomb and of the adjoining perforated sheets used for constructing flow panels in such earlier devices were unnecessarily thick, therefore creating large acoustic dead spaces where individual panels join.
Another problem encountered during static testing of jet engines is engine speed instability due to wind gusts and atmospheric turbulence. Large turbofan engines are sensitive to inflow distortion, the result being fluctuations in the engine speed which are difficult to prevent.
In view of the foregoing, it is the broad purpose and object of the present invention to provide a device which reduces flow distortions in air entering the intake of a jet engine during static tests.
It is also an object of this invention to provide a device which reduces both streamwise and transverse components of inflow turbulence in the air entering jet aircraft engines during static noise tests.
It is a further object of this invention to provide an efficient, lightweight turbulence control device for jet engine noise testing which has no objectionable sound obstructing or reflecting supporting members foward of the intake of the engine, and which requires a minimum support structure rearward of the engine intake.
It is a further object of this invention to provide a turbulence control device, the supporting framework of which occupies or obstructs a minimum surface area normal to the mean airflow at the control surface.
It is a further object of this invention to provide a device, the panels and panel joints of which minimize the reflection and diffraction of emitted engine intake noise.
It is a further object of this invention to provide a device which stabilizes the engine speed of a jet engine during static testing.